This patch adds support for the Sega Dreamcast visual memory unit as aflash mapping. It requires changes in the maple bus driver (postedseparately) to support block reads and writes.

The VMU is a 'smart' flash device, with a built-in 8-bit controller -for instance there is an erase before a write but it is hidden from theuser. But the device's overall behaviour means it works well with themtd layer and it is appropriate to add it as an mtd mapping.